1. Field of the Invention
The present invention relates to an X-ray CT scanner including a two-dimensional detector that has a plurality of columns (a plurality of segments) thereof laid out in the direction of the axis of rotation of a gantry (in the direction of a center axis about which an X-ray focal spot is rotated, or simply, the direction of the thickness of a slice plane (slice thickness)), and that has the widths of the plurality of columns thereof defining slice thicknesses (slice pitches) made unequal.
2. Description of the Related Art
X-ray CT scanners include a fan-beam (single-slice) X-ray CT scanner or a type of X-ray CT scanner that has been adopted in the past.
The fan-beam X-ray CT scanner has an X-ray source and detector opposed to each other with a subject (for example, a patient) between them. The detector has detecting elements, which constitute, for example, approximately 1000 channels, arranged in the form of a sector in a (channel) direction orthogonal to a body-axial direction of the subject.
In the X-ray CT scanner, a fan-shaped X-ray beam is irradiated from the X-ray source to a certain slice plane (or, simply, a slice) of the subject. An X-ray beam transmitted by the slice plane of the subject is detected by the detector, and then X-ray transmission data is acquired.
The acquired X-ray transmission data is sent to a data acquisition system (DAS) having elements associated with the detecting elements of the detector. Each element carries out amplification or the like and acquires projection data (one data acquisition is referred to as one view).
While the X-ray source and detector are rotated in unison about the subject, X-rays are irradiated and data acquisition is repeated approximately 1000 times. Consequently, projection data in multiple directions of the subject is acquired. Based on the projection data in multiple directions, the image of the slice plane of the subject is reconstructed.
In such a single-slice X-ray CT scanner, the image of a certain slice plane of a subject is produced. It is therefore hard to produce images of a wide range of the subject for a short period of time. There is therefore an increasing demand from doctors and the like for producing high-definition (high-resolution) images of a wide range of a subject for a unit period of time.
In an effort to meet the demand, studies have been made on a multi-slice X-ray CT scanner in recent years.
The multi-slice X-ray CT scanner has a plurality of columns (a plurality of (N) segments) of detectors, each of which is the same as the one employed in the single-slice X-ray CT scanner, in the body-axis direction of a subject (also referred to as a slice-thickness direction or segment direction). The detectors constitute a two-dimensional detector having detecting elements numbering the product of M channels by N segments. In this case, elements of a DAS are associated with the detecting elements of the two-dimensional detector.
In other words, the multi-slice X-ray CT scanner has an X-ray source for bombarding a conical X-ray beam, and the foregoing two-dimensional detector. X-rays of the conical X-ray beam (diameter of an effective field of view, FOV) passing through a subject are detected by the two-dimensional detector, whereby projection data of multiple slice planes of the subject is acquired at a time. Thus, the multi-slice X-ray CT scanner is expected to enable acquisition of high-definition images from a wide range.
Various proposals have been made of the configurations of such a multi-slice X-ray CT scanner and two-dimensional detector employed in the multi-slice X-ray CT scanner.
For example, known is an idea of freely changing one slice thickness by combining X-ray data items detected by a plurality of segments through image post-processing based on detected data.
Thinking of the specifications of a two-dimensional detector and DAS for a multi-slice X-ray CT scanner, several parameters have significant meanings. To be more specific, for improving the resolution in a body-axis direction, it is necessary to finely set the pitches in the body-axis direction of elements corresponding to segments of the detector (slice thickness) relative to adjoining ones. For expanding a scanned region in the body-axis direction (for eventually shortening the scan time of a certain region), the size of the whole detector (the number of the columns corresponding to the segments of the detector) must be made larger. In an effort to clear both the requirements that are seemingly contradictory, that is, improvement of the resolution in the body-axis direction and expansion of a scanned region, it has been conceived that sufficiently small detecting elements that are fine divisions of a detector are arranged in the body-axis direction by the number of columns (segments) defining a sufficiently large size.
However, on the detector side, there are limitations in a minimum size of an element (in a slice-thickness direction) and a maximum number of elements because of the problems that geometrical efficiency is deteriorated with finer segmentation of the detector and that the density of wiring patterns increases with an increase in number of elements. It is therefore currently thought that approximately 1 mm and approximately 30 columns are feasible levels of the minimum size of an element and of the maximum number of columns of elements respectively.
For arranging approximately 30 columns of detecting elements, it is necessary to install a DAS having the number of elements corresponding to the number of segments or columns of the detecting elements. A simple countermeasure is to arrange a plurality of (30) columns of currently-employed DASs. In reality, there are limitations in the number of elements of a DAS that can be arranged because of the problem of preserving an installation space in a scanner system or the problem of ensuring appropriate cost performance. The existing high-density installation technology and manufacturing cost permit about 10 columns of elements as a level feasible in the near future.
Since restrictions are thus placed differently on the parameters such as the number of elements of a DAS, a minimum size of an element of a detector, and a maximum number of elements in the detector, it is hard to attain high resolution in the body-axis direction and a wide scanned region by nonchalantly combining these parameters. A further commitment to novelties and improvements is requested.
FIGS. 28A and 28B show examples of a combination of parameters according to a prior art, that is, examples of a row of detecting elements of a detector constituting one channel in such a connection mode that a detector composed of 30 columns of detecting elements having an equal pitch (slice thickness) of 1 mm or 2 mm relative to adjoining ones and a DAS having elements that numbers a multiple of 10 slices are combined via a group of switches. FIG. 28A shows a resolution-conscious structure in which the width of a detecting element is set to 1 mm, while FIG. 28B shows a scanned region-conscious structure in which the width of a detecting element is set to 2 mm.
In the resolution-conscious structure, as shown in FIGS. 28Aa, the detector has 30 segments of 1 mm wide arranged. Since the DAS has elements that numbers a multiple of 10 slices, data acquisition can be carried out in the range from data acquisition of a total of 10 slices having a pitch of 1 mm relative to adjoining hones (1 mm.times.10 slices=10 mm)(See FIGS. 28Ab) to data acquisition of a total of 10 slices having a pitch of 3 mm relative to adjoining ones (3 mm.times.10 slices=30 mm)(See FIG. 28Ac).
In the resolution-conscious structure, the resolution in a segment direction can be made as fine as 1 mm. However, the scanned region is limited to a maximum of 30 mm or so. A sufficiently wide scanned region is thus unavailable.
By contrast, in the scanned region-conscious structure, as shown in FIGS. 28Ba, the detector has 30 segments of 2 mm wide arranged. Like the resolution-conscious structure, data acquisition can be carried out in the range from realization of a total of 10 slice having a pitch of 2 mm relative to adjoining ones (2 mm.times.10 slices=20 mm)(See FIG. 28Bb), to realization of a total of 10 slices having a pitch of 6 mm relative to adjoining ones (6 mm.times.10 slices=60 mm)(See FIG. 28Bc).
In the scanned region-conscious structure, the scanned region is 60 mm at maximum, and thus a sufficiently wide scanned region is provided. However, a minimum slice pitch in the segment direction is as large as 2 mm. Sufficient resolution is unavailable.
For example, when the size of an element in a detector is set to a large value with emphasis placed on a wide scanned region, a desired slice thickness may not be able to be attained. For example, when numerous detecting elements that are segments of 2 mm wide are arranged, slices having, for example, a pitch of 5 mm relative to adjoining ones cannot be specified even by changing the setting of the group of switches designed for combining signals.
As mentioned above, increasing the numbers of detecting elements and of elements of a DAS is preferable for realizing high resolution in the body-axis direction and a wide scanned region. However, there is the fear that an increase in the number of elements may lead to deterioration of reliability of a whole scanner. In consideration of recent technological advancement, it cannot always be said that a failure rate increases in proportion to an increase in the numbers of detecting elements and of elements of a DAS. For limiting the failure rates of a detector having detecting elements numbering a value that is, for example, 30 times as large as the number of detecting elements included in an existing detector and a DAS having elements numbering a value that is, for example, 10 times as large as the number of elements included in an existing DAS to the same failure rates of the existing detector and DAS, reliabilities that are 30 times and 10 times higher than those of the existing detector and DAS are required. Specifically, for increasing the numbers of elements of a detector and elements of a DAS, it is necessary to devise a means for maintaining reliability in case a failure occurs at a rate proportional to the increase.